Conventional capacitive elements for high frequency devices or for decoupling are capacitive elements having a PIP (polysilicon/insulating film/polysilicon) structure in which polysilicon is used for both, upper and lower, electrodes or a MOS (polysilicon electrode/gate silicon oxide film/silicon substrate) structure. However, polysilicon electrodes have problems such as large resistance and depletion. MIM (metal/capacitive insulating film/metal) structures are employed in which a metal or metal oxide film, such as titanium nitride and ruthenium oxide, is used for the electrodes. To date, titanium oxide films are extensively studied for the electrodes in MIM structures. This is because the electrodes in an MIM structure have low electric resistance and easy to form by etching.
Recently, as LSIs become smaller and highly integrated, MIM capacitors have a reduced occupying area. Then, there is an increasing demand for a capacitor having a large capacitance and a small area. For this reason, in place of conventional capacitive insulating film structures using silicon oxide films or gate oxide films, usage of high dielectric constant (High-k) materials such as high relative dielectric constant metal oxide films (for example, TaO, HfO, and ZrO) and metal silicate films (for example, HfSiO) as capacitive insulating films has been discussed. However, even if a High-k material is used, the insulating film has to have a thickness reduced to 20 nm or smaller in order to realize a capacitive density of 10 fF/μm2 or higher, which is required in LSIs having a line width or 65 nm or smaller.
Metal oxide films generally have a high relative dielectric constant but a problem is that they are accompanied by high current leakage. Heat treatment following formation of a metal oxide film causes crystallization, which may increase current leakage running through the crystal grain boundary. Conversely, if no heat treatment is performed, insufficient oxidization causes oxygen defects, which provide leakage paths and, again, may increase current leakage. When the metal materials are transition metals, their bonding force to oxygen is weak and, therefore, some reduction damage may occur in processes following formation of an insulating film, by which current leakage is further increased. On the other hand, metal silicate films have significantly lower relative dielectric constants than metal oxide films. However, it is known that since they contain silicon oxides, metal silicate films are thermally more stable and have higher crystallization temperatures than films of a metal oxide alone, thereby allowing for low current leakage (for example, see Patent Literature 3). Furthermore, silicon binds to oxygen with a strong bonding force and is highly resistant to reduction damage. Therefore, transition metal silicate films are expected to improve damage resistance.
As for problems with High-k materials, metal oxide films as a capacitive insulating film have poor insulation properties and damage resistance and metal silicate films have significantly low relative dielectric constants. Films of High-k materials are generally formed by ALD (atomic layer deposition), sputtering, CVD (chemical vapor deposition), and the like. The above problems occur in any of the formation methods. Therefore, extensive efforts have been made to develop a High-k insulating film exhibiting low current leakage properties while maintaining a high relative dielectric constant. Patent Literature 1 to 3 below disclose techniques relating to the present invention.
Patent Literature 1 describes a method of realizing a low leak insulating film by preventing oxygen defects occurring in the electrode/insulating film interface. An antioxidant film and an insulating film are successively formed by ALD without exposing to the atmosphere after a lower electrode is formed, by which the insulating film is formed on a clean underlying layer surface, preventing oxygen defects from occurring in the electrode/insulating film interface. With the oxygen defects being significantly reduced without crystallization, a low leak metal oxide film having a high relative dielectric constant can be realized.
Patent Literature 2 describes a method using an alloyed metal oxide film for preventing oxygen defects occurring in High-k insulating films. Two materials are mixed by ALD to form an alloyed metal oxide film (for example, a TaTiO film), by which crystallization in an oxide film is prevented as in a metal silicate film and oxygen defects in a High-k insulating film can be prevented by heat treatment. Using metals having high relative dielectric constants, low current leakage properties can be realized without lowering the dielectric constant.
Patent Literature 3 describes a method of realizing high capacitance while preventing oxygen defects by manipulating the distributions of silicon and metal in the thickness direction of a metal silicate film. A metal silicate film is formed in the manner that the silicon composition ratio is higher than the metal composition ratio near the interface compared with in the inner part in the thickness direction of the metal silicate film, by which the metal silicate film is nearly a silicon oxide film (SiO2) and is highly insulating near the interface where current leakage is rate-controlled. The metal silicate film is nearly a metal oxide film and has a high relative dielectric constant in the inner part. Therefore, a High-k insulating film having a higher insulation property and a higher relative dielectric constant than conventional metal silicate films can be realized.
Patent Literature 1: Unexamined Japanese Patent Application KOKAI Publication No. 2007-129190;
Patent Literature 2: Unexamined Japanese Patent Application KOKAI Publication No. 2001-053254; and
Patent Literature 3: Unexamined Japanese Patent Application KOKAI Publication No. 2006-054382.